Here is how it looks like. I was able to capture it via the viewfinder using my phone. Note the area above the top focus point:

Not to worry. I'm an expert for this job. Red or Black?

This is sounding more and more like the 5d3 light leak 'issue', that wasn't really an issue. Only thing I can think this would affect is metering, and that's if the light stays on and it's low light. Maybe some light leaking from the view finder down into the sensor, but it'd have to be a pretty long exposure with very low light for that. If it stays on.

Umm... No. This is a reflection in the viewfinder area where the focusing screen and focus points exist, not the sensor area. Verify this by seeing if the problem exists in liveview. Wait, those points don't show up in live view because....

Umm... No. This is a reflection in the viewfinder area where the focusing screen and focus points exist, not the sensor area. Verify this by seeing if the problem exists in liveview. Wait, those points don't show up in live view because....

Right, but the light can, possibly get through the AF partial-mirror on the main mirror and hit the sensor. On a fast exposure or with lots of light it likely won't make a difference, but in a long exposure (>1sec) it may. Also, it may affect metering, if the red light doesn't turn off when the camera determines the final metering when the shutter button is pressed.

Right, but the light can, possibly get through the AF partial-mirror on the main mirror and hit the sensor. On a fast exposure or with lots of light it likely won't make a difference, but in a long exposure (>1sec) it may.

The AF illumination light would be shut off during the exposure anyway.

Even if the light was on during an exposure there is no way that little amount of extra light would affect the exposure. If that was enough light to throw off the exposure then the the AF point light would already be throwing it off. But, as neuro said, the lights are off then anyway.